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No annual maintenanceAll HVAC systems need an annual cleaning and tuning, which is typically done in the fall by your heating professional or fuel provider. "You wouldn't let your car go more than a year without a checkup, would you?" says Richard. Homeowners can easily remove a register and vacuum out vents in the floor (particularly kitchen vents) at least once a year. Seriously filthy ductwork may need professional cleaning.

Oversized heating systems"No contractor wants to hear a customer say, 'I don't have enough heat,'" Richard says. "So they install a boiler or furnace big enough for the coldest day of the year, plus a safety factor of 25 to 50 percent. But the coldest day of the year happens for just a handful of hours. So now you've paid for this appliance that's way too big for 99.9 percent of the time. That means more frequent on-off cycling, which wears out the unit and invites incomplete combustion."

Richard's solution: Make sure the contractor performs a heat-loss calculation—a measure based on a house's insulation levels, airtightness, and the local climate—and orders equipment that meets peak demand without any additional safety factor. The same goes for cooling equipment.

Not installing high-efficiency filtersRichard favors the thick, pleated media filters with antimicrobial coatings. "All the air you breathe comes through that filter," he says. "It makes no sense to use the cheap ones you can see through." Replace them annually when the system is being serviced.